Lubricity of fluorinated alkyl aryl ethers

Article abstract of DOI:10.1016/S0022-1139(99)00293-6

A series of fluorinated alkyl aryl ethers, composed of hydrocarbon components, fluorinated alkyl groups, and ether linkage groups, were evaluated through a variety of tests to investigate lubricating properties. The basic molecular structure with tetrafluoroethoxyphenyl groups were shown to have excellent lubricities. However, ortho-position substituents to ether linkage groups diminished extreme-pressure properties. Excellent lubricities were suggested to be due to strong coordination of ether oxygen atoms to metal surfaces, and formation of iron fluorides on the metal wear surfaces.

Full text of DOI:10.1016/S0022-1139(99)00293-6

Journal of Fluorine Chemistry 103 (2000) 129±134
Lubricity of ¯uorinated alkyl aryl ethers
Hiroyuki Fukui*, Hiroshi Murata, Ken-ichi Sanechika, Masanori Ikeda
Asahi Chemical Industry Co., Ltd., Corporate R&D Administration, 1-1-2 Yuraku-cho, Chiyoda-ku, Tokyo, Japan
Received 7 July 1999; received in revised form 27 September 1999; accepted 5 November 1999
Abstract
A series of ¯uorinated alkyl aryl ethers, composed of hydrocarbon components, ¯uorinated alkyl groups, and ether linkage groups, were
evaluated through a variety of tests to investigate lubricating properties. The basic molecular structure with tetra¯uoroethoxyphenyl groups
were shown to have excellent lubricities. However, ortho-position substituents to ether linkage groups diminished extreme-pressure
properties. Excellent lubricities were suggested to be due to strong coordination of ether oxygen atoms to metal surfaces, and formation of
iron ¯uorides on the metal wear surfaces. # 2000 Elsevier Science S.A. All rights reserved.
Keywords: Lubricity; Metal ¯uoride; Fluorinated alkyl aryl ethers; Synthetic lubricants
1. Introduction
2.1.1. Synthesis of 2,2-bis[4-(1,1,2,2-
tetrafluoroethoxy)phenyl]propane
(hereinafter referred to as `BisA-TFE')
As performance and energy ef®ciency demands on
mechanical apparatus are increasingly heightened, require-
ments for lubricant performance are becoming correspond-
ingly demanding. In a previous study [1], a series of novel
¯uorinated alkyl aryl ethers developed as candidates for base
stock of high performance lubricants were evaluated for
thermal and oxidative stability. It was shown that the basic
molecular structure of the ¯uorinated alkyl aryl ethers had
excellent oxidative and thermal stability, making them and
their derivatives suitable for further investigation as candi-
dates for base stock of high performance lubricants. Due to
the presence of ¯uorine atoms, these compounds were
expected to intrinsically have excellent lubricity. In this
study, lubricity of the series of ¯uorinated alkyl aryl ethers
was investigated.
A solution consisting of 68.7 g of 2,2-bis(4-hydroxyphe-
nyl)propane (hereinafter referred to simply as `bisphenol A')
and 6.2 g of potassium hydroxide dissolved in 120 ml of
dimethyl sulfoxide was charged into a 500 ml microcylin-
der, which served as the reactor. The reactor was degassed,
charged with N2 gas to restore atmospheric pressure,
and then heated to 608C using an oil bath. Tetra¯uoro-
ethylene was then introduced to initiate a reaction. The
reaction was carried out for about 5 h with tetra¯uoroethy-
lene fed in to maintain reactor pressure between 3 and
4 kg/cm2.
After completion of the reaction, reaction product was
obtained by distilling off dimethyl sulfoxide at 808C under
about 5 mm Hg, and 500 ml of 1,1,2-trichloro-1,2,2-tri¯uor-
oethane (hereinafter referred to simply as `CFC-113') was
added to the obtained reaction product. This CFC-113 phase
was washed three times with 500 ml of distilled water, and
CFC-113 was removed under reduced pressure. Thus, 130 g
of colorless, transparent oil was obtained. The main fraction
(118 g) was isolated by simple distillation, at a boiling point
of about 1508C under about 0.1±0.3 mm Hg, indicating that
the synthesis reaction yield was 92%.
2. Experimental
2.1. Synthesis of fluorinated alkyl aryl ethers
The ¯uorinated alkyl aryl ethers investigated in this study
were synthesized as follows.
The isolated fraction was analyzed by infrared absorption
‡
spectrometry and mass spectrometry [m/e 428 (M ), 413
‡
* Corresponding author. Tel.: ‡81-3-3507-2234; fax: ‡81-3-3507-2426.
E-mail address: a8411115@ut.asahi-kasei.co.jp (H. Fukui).
(M ±CH3)] to con®rm that the fraction was BisA-TFE,
having the structural formula shown in Table 1.
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 9 3 - 6

130
H. Fukui et al. / Journal of Fluorine Chemistry 103 (2000) 129±134
Table 1
Lubricity of basic fluorinated alkyl aryl ether and conventional high performance lubricants
Sample
Viscositya (cSt at 408C)
Coefficient of frictionb
Four-ball testc
Fail load (kg/cm2)
Falex testd
Fail load (kgf)
Wear (mg)
BisA-TFE
Naphthene mineral oilf
Ester oilh
Turbine oili
26
30
20
32
0.13
0.15
0.14
0.15
9.0
5.0
5.0
4.5
>680
180
±
14.2e
40g
±
±
±
a Measured with E-type rotational viscometer manufactured by Tokyo Keiki K.K.
b Measured with pendulum-style friction tester manufactured by Shinko Engineering.
c Measured with Soda four-ball test machine manufactured by Shinko Engineering.
d Measured with Falex pin and vee-block test machine manufactured by Shinko Engineering.
e Measured under blowing of HFC-134a.
f Suniso 3GS, manufactured by Nippon Sun Oil Company.
g Measured under blowing of CFC-12.
h Unistar H334R, manufactured by Nippon Oil Fats.
i Tellus Oil 32, manufactured by Showa Shell Sekivu K.K. (containing additives).
2.1.2. Synthesis of 1,1,3,3-tetramethyl-4-(1,1,2,2-
tetrafluoroethoxy)phenylbutane (hereinafter referred
to as `PTOP-TFE')
that the products were compounds having the structural
formulas shown in Table 2.
PTOP-TFE was prepared in substantially the same man-
ner as BisA-TFE except that p-tert-octylphenol (obtained
from Tokyo Kasei Kogyo) was used in place of bisphenol A.
The synthesis reaction yield was 94%. Infrared absorption
spectrometry and mass spectrometry [m/e 306 (M )] con-
®rmed that the obtained product was PTOP-TFE, having the
structural formula shown in Table 2.
2.2. Conventional lubricants
Commercially available lubricants were used in this study
to provide reference data. Naphthene mineral oil (Suniso
3GS) was obtained from Nippon Sun Oil Company, ester oil
(Unistar H334R) was obtained from Nippon Oil Fats and
turbine oil (Tellus Oil 32) was obtained from Showa Shell
Sekiyu K.K. Three types of per¯uoropolyether oils, as
below, were also studied.
‡
2.1.3. Synthesis of 2,2-bis[4-(1,1,2,2-
tetrafluoroethoxy)phenyl]-4-methylpentane
(hereinafter referred to as `MIBK-Bis-TFE')
Fluid K: C3F7O[CF(CF3)CF2O]xC2F5
Fluid F: CF3O[CF(CF3)CF2O]m(CF2O)nCF3
Fluid D: C3F7O(CF2CF2CF2O)xC2F5
MIBK-Bis-TFE was prepared in substantially the same
manner as BisA-TFE except that 2,2-bis(4-hydroxyphenyl)-
4-methylpentane (obtained from Honshu Chemical Indus-
try) was used in place of bisphenol A. The synthesis reaction
yield was 95%. Infrared absorption spectrometry and mass
Fluid K was obtained from Du Pont (Krytox 143AB),
Fluid F was obtained from Monte¯uos (Fomblin Y25A) and
Fluid D was obtained from Daikin Industries (Demnum S-
65).
‡
spectrometry [m/e 470 (M )] con®rmed that the obtained
product was MIBK-Bis-TFE, having the structural formula
shown in Table 2.
2.3. Evaluation of lubricating properties
2.1.4. Synthesis of BisP-OT-TFE, BisP-IOTD-TFE,
BisC-TFE and BisOSBP-A-TFE
Coef®cient of friction was measured using a pendulum-
style friction tester manufactured by Shinko Engineering
with an initial oil temperature of 208C. Four-ball test fail
load was measured using a Soda four-ball test machine
manufactured by Shinko Engineering at 200 rpm. Load oil
pressure was increased by 0.5 kgf/cm2/min, and fail load
was determined as the oil pressure at seizure.
These compounds were prepared in substantially the
same manner as BisA-TFE, except that 2,2-bis(4-hidroxy-
phenyl)octane, 1,1-bis(4-hydroxyphenyl)-2-ethylhexane,
2,2-bis(3-methyl-4-hydroxyphenyl)propane, and 2,2-bis(3-
sec-butyl-4-hydroxyphenyl)propane, respectively, were
used in place of bisphenol A. Each of these materials were
obtained from Honshu Chemical Industry. Synthesis reac-
tion yields were 90, 88, 90, and 94%, respectively. Infrared
absorption spectrometry and mass spectrometry con®rmed
Falex test fail load1 was measured using a Falex pin and
vee-block test machine manufactured by Shinko Engineer-
1 ASTM Designation D3233-86.

H. Fukui et al. / Journal of Fluorine Chemistry 103 (2000) 129±134
131
Table 2
Lubricity of substituted fluorinated alkyl aryl ethers
Compound
Viscosity (cSt at 408C)a
Falex testb
Fail load (kgf)
Wear (mg)
PTOP-TFE
6
109
112
250
115
91
360
>680
>680
460
10.0c
17.7c
12.8c
14.2c
±
14.7c
14.2c
40.0e
MIBK-Bis-TFE
BisP-OT-TFE
BisP-IOTD-TFE
BisC-TFE
BisOSBP-A-TFE
480
440
cf. BisA-TFE
cf. Naphthene mineral oild
26
>680
180
30
a Measured with E-type rotational viscometer manufactured by Tokyo Keiki K.K.
b Measured with Falex pin and vee-block test machine manufactured by Shinko Engineering.
c Measured under blowing HFC134a.
d Suniso 3GS, manufactured by Nippon Sun Oil Company.
e Measured under blowing CFC-12.
ing. The tester was driven for 3 min at a load of 136 kgf,
then load was increased continuously until failure.
Wear was measured using a Falex pin and vee-block test
machine2. Refrigerant gas (CFC-12 or HFC-134a) was
blown into sample oil at a blow rate of approximately
150 ml/min for about 30 min. A load of 90 kgf was applied,
and the Falex tester was driven for 5 min with continuous
gas blowing. Load was then increased to 230 kgf, and the
tester was driven for 2 h under constant 230 kgf load. The
weight of the vee-block and pin was measured before and
after each test run, and `wear' was de®ned as the difference
between these weight measurements.
Four-ball extreme-pressure tests3 were carried out using a
tester manufactured by Falex. A series of tests of 60 s
duration were conducted at 1500 rpm with increasing load
(20, 32, 40, 50, 63, 80, 100, 160, and 200 kgf) until welding
occurred. The scar diameters of the stationary test balls were
measured, and the `last non-seizure load' was de®ned as the
lowest applied load at which the measured scar diameters
were not more than 5% above the reference scar diameter as
calculated to compensate for dynamic conditions speci®c to
the test apparatus.
2.4. XPS measurement
Journals from Falex wear tests were rinsed with acetone
to remove remaining lubricant, and analyzed by X-ray
photoelectron spectrometry (XPS) using an ESCA Lab.
200X manufactured by VG Scienti®c.
2.5. Measurement of total acid number (TAN)
TAN was measured in substantially the same manner as
ISO 6618, except that 0.01 mol/l solution of potassium
hydroxide was used in place of 0.1 mol/l solution.
3. Results and discussion
3.1. Lubricating properties of basic fluorinated alkyl aryl
ether compound
Lubricating properties of BisA-TFE and conventional
high performance lubricants (naphthene mineral oil, ester
oil and turbine oil) are shown in Table 1. BisA-TFE had the
lowest coef®cient of friction, at 0.13, and the highest four-
ball test fail load, at 9 kg/cm2.
2 ASTM Designation D2670-88.
3 ASTM Designation D783-88 (Reproved 1993).
Falex fail load and wear tests were carried out to further
investigate BisA-TFE performance in comparison with

132
H. Fukui et al. / Journal of Fluorine Chemistry 103 (2000) 129±134
naphthene mineral oil. Naphthene mineral oil is widely used
as base oil for refrigeration lubricants in refrigerating
machines using conventional chloro¯uorocarbon (CFC)
refrigerant. Though the lubricity of naphthene mineral oil
alone is not suf®cient, it improves substantially when metal
chlorides form on metal surfaces with chlorine atoms from
CFC refrigerants [2,3]. To compensate for this effect in the
Falex wear test, 1,1-dichloro-1,1-di¯uoromethane (CFC-
12) was blown into the sample oil during testing.
In order to measure the performance of BisA-TFE with-
out the metal chloride effect, its Falex wear test was
conducted without CFC refrigerants. However, in order to
attain test results comparable with those of naphthene
mineral oil, 1,1,1,2-tetra¯uoroethane (HFC-134a) was
blown into the BisA-TFE during testing to reproduce any
reduction in viscosity which may result from dissolution of
refrigerant gas in the sample oil, as well as any detrimental
effect on lubricity which may result from the presence of
refrigerant bubbles on the metal wear surfaces.
Falex test results are also shown in Table 1. The fail load
for naphthene mineral oil was 180 kgf, but no fail load for
BisA-TFE was observed up to the test limit 680 kgf. The
wear for BisA-TFE was 14.2 mg, while that for naphthene
mineral oil was 40 mg. Considering that the lubricity of the
naphthene mineral oil was improved by the effect of metal
chloride formation, the lubricity of BisA-TFE was clearly
superior.
3.2. Relationship between lubricity and the molecular
structure
Table 2 shows Falex test results for various ¯uorinated
alkyl aryl ethers. Tests were conducted with HFC-134a
blown in sample oils. Fail load and wear results varied
somewhat, but lubricity of all ¯uorinated alkyl aryl ethers
were clearly superior to that of naphthene mineral oil.
The fail load for PTOP-TFE was the lowest among the
¯uorinated alkyl aryl ethers, at 360 kgf. It has been shown
that dihydric alcohols have better lubricating properties than
monohydric alcohols, because of the stronger coordination
to metal surfaces which results from the greater number of
coordination sites [2,5]. The authors believe that similarly,
BisA-TFE, with two ether groups, coordinates more
strongly to metal surfaces than PTOP-TFE, which has only
one ether group. With only one coordination site per mole-
cule, PTOP-TFE forms a weaker ®lm than BisA-TFE which
has two sites per molecule. The relative weakness of the ®lm
then explains the lower fail load result.
Bis-MIBK-TFE and BisP-OT-TFE are compounds with
an iso-butyl group and an n-hexyl group, respectively,
substituted on the central carbon of BisA-TFE. As with
BisA-TFE, the fail loads of both exceeded the 680 kgf limit
of the test apparatus, indicating that alkyl group substitution
on the central carbon had no observable effect on extreme-
pressure properties.
It is believed that the excellent lubricity of BisA-TFE can
be explained by examining its molecular structure, as illu-
strated in Fig. 1. The low coef®cient of friction is attributed
to weak intermolecular forces between the ¯uorine atoms of
the Rf groups. The outstanding extreme-pressure properties
are attributed to the strong coordination of the ether oxygen
atoms to fresh metal surfaces. It has been reported that under
boundary lubrication conditions, metal oxides and adsorbed
impurities are removed from asperity contacts on the wear
surfaces [4]. The fresh metal surfaces thus exposed are
believed to enable strong ether oxygen coordination, result-
ing in very high strength lubricant ®lm. These character-
istics of the BisA-TFE molecule are expected to be common
to all derivative species, barring the effects of substituent
groups.
BisP-IOTD-TFE, on the other hand, gave a fail load result
of 460 kgf. As reported previously [1], BisP-IOTD-TFE has
relatively poor thermal and oxidative stability. Decomposi-
tion by heat generated from friction is believed to explain
the low extreme-pressure property of BisP-IOTD-TFE.
BisC-TFE and BisOSBP-A-TFE are compounds with
methyl groups and sec-butyl groups, respectively, substituted
at ortho-positions with the ether linkagegroups of BisA-TFE.
Their fail loads were signi®cantly lower than that of BisA-
TFE, at 480 and 440 kgf, respectively. It is believed that these
compounds have inferior extreme-pressure performance
because ortho-position substituents prevent the coordination
of ether oxygen atoms to the metal surface, in contrast to
substituents on the central carbon which do not prevent such
coordination, as illustrated in Fig. 2.
Fig. 1. Suggested mechanism of lubricity of BisA-TFE.
Fig. 2. Effects of substituent group position on extreme pressure lubricity.

H. Fukui et al. / Journal of Fluorine Chemistry 103 (2000) 129±134
133
Table 4
XPS analysis of journal surfacea
These results support the suggested mechanism
described above, that the strong coordination of the ether
oxygen atoms to metal surfaces results in the formation
of lubricant ®lm which is very durable under extreme-
pressure.
Specimen
Lubricant
Refrigerant gas
blown into lubricant
Wear
(mg)
Sample A
Sample B
BisOSBP-A-TFE
BisOSBP-A-TFE
HFC-134a
None
1.7
0.9
Wear results in the range of 10.0±17.7 mg were
observed for all ¯uorinated alkyl aryl ethers, much less
than that for naphthene mineral oil. The relationship
between wear results and steric effects of substituents
was not clear. As wear test load was 230 kgf, much lower
than the fail loads, and wear test period was 2 h, much
longer than the fail load tests, other properties of lubricants,
for example, viscosities and thermal stabilities, are pre-
sumed to have in¯uenced wear results. Further investiga-
tions by authors will be concerned with the relationship
between wear properties and molecular structures of ¯uori-
nated alkyl aryl ethers.
a Test conditions Ð Tester: Falex pin and vee-block test machine,
manufactured by Shinko Engineering; Starting temperature: 808C; Load:
68 kgf; Test duration: 120 mm.
3.4. XPS analysis of wear surface
The formation of ferric ¯uoride (FeF3) through the inter-
action of per¯uoropolyethers with steel surfaces under
boundary conditions has been well researched [6±9] and
it is known that iron ¯uoride act as wear reducer [10].
Falex journal specimens for XPS analysis of iron ¯uoride
formation with BisOSBP-A-TFE were prepared as shown in
Table 4. Fig. 3 shows a wide scan XPS spectrum of the
surface of sample A. Narrow scan XPS spectrum of sample
A (Fig. 4) indicates that the signal for ¯uorine was at
684.2 eV. This is very near to the 685 eV signal indicating
FeF3 which has been observed on the wear surface using
per¯uoropolyether as lubricant [7]. The 684.2 eV signal is
therefore believed to indicate the formation of iron ¯uorides
on the wear surface in the present test.
As sample A was a journal from a Falex test conducted
with HFC-134a, the ¯uorine atoms of the iron ¯uoride may
have originated from either the BisOSBP-A-TFE or the
HFC-134a. However, in the narrow scan XPS spectrum of
sample B, a journal from a Falex test conducted without
HFC-134a, an iron ¯uoride signal was observed at same
position (Fig. 4), indicating that ¯uorine from the ¯uori-
nated alkyl aryl ether does form iron ¯uorides on the wear
surface.
3.3. Lubricity comparison with perfluoropolyethers
Table 3 shows lubricity of ¯uorinated alkyl aryl ethers
and per¯uoropolyethers, which also contain ¯uorinated
alkyl groups and ether linkage groups, as evaluated by
extreme-pressure four-ball tests.
Three types of per¯uoropolyether were evaluated. Their
last non-seizure loads were 20 and 32 kgf. Last non-seizure
loads for BisOSBP-A-TFE and BisP-OT-TFE were higher
than those of the per¯uoropolyethers, at 50 kgf, indicating
that the load range where seizure does not occur is clearly
broader with the ¯uorinated alkyl aryl ethers than per¯uor-
opolyethers. Due to weak basicity of their ether oxygen
atoms, per¯uoropolyethers are not expected to form strong
lubrication ®lm. Therefore, local seizure will be caused at
the asperity contacts under boundary lubrication conditions.
In contrast, the strong lubrication ®lm of the ¯uorinated
alkyl aryl ethers prevents local seizure, resulting high last
non-seizure loads.
It is believed that the formation of iron ¯uorides with
¯uorinated alkyl aryl ethers may be explained by the
coordination of ether oxygen atoms to the metal surface,
which would place the ¯uorinated alkyl groups very close to
the metal surface. A reaction involving the ¯uorinated alkyl
Table 3
Lubricity of fluorinated alkyl aryl ethers and perfluoropolyethers
Compound
Viscositya
Last non-seizure
loadb (kgf)
(cSt at 408C)
BisOSBP-A-TFE
BisP-OT-TFE
112
91
50
50
Perfluoropolyether
Fluid Kc
85
94
65
20
32
32
Fluid Fd
Fluid De
a Measured with E-type rotational viscometer manufactured by Tokyo
Keiki K.K.
b Measured with four-ball extreme-pressure tester manufactured by
Falex.
c Krytox 143AB, manufactured by E.I. DuPont De Nemours and
Company.
d Fomblin Y25A, manufactured by Nippon Montedison K.K.
e Demnum S-65, manufactured by Daikin Industries.
Fig. 3. Wide scan XPS spectrum of sample A.

134
H. Fukui et al. / Journal of Fluorine Chemistry 103 (2000) 129±134
series studied results in excellent lubricating properties, (2)
compounds obtained by substitutions on the basic molecular
structure also have excellent lubricity, but ortho-position
substituents diminish extreme-pressure properties, (3)
excellent extreme-pressure properties are believed to be
due to the high lubricant ®lm strength which results from
strong coordination of ether oxygen atoms to metal surfaces,
and (4) ¯uorine atoms from the ¯uorinated alkyl groups
form iron ¯uorides on the metal wear surfaces, which
improve wear properties.
These conclusions will serve as a useful guide to design-
ing novel high-performance lubricants. In subsequent stu-
dies, the authors will evaluate practical lubricating
properties of this series of ¯uorinated alkyl aryl ethers
and lubricants derived from them.
Acknowledgements
Fig. 4. Narrow scan XPS spectra of samples A and B.
The authors are grateful to Dr. Jun Kikuma , Mr. Masashi
Watanabe and Mr. Atsushi Seo of Asahi Chemical for their
helpful discussion.
groups and metal atoms at the surface would then occur by
heat of friction, resulting in iron ¯uoride formation, which
improves wear properties.
Iron ¯uoride also catalyzes the scission of carbon±oxygen
bonds in per¯uoropolyethers, resulting in degradation pro-
ducts and polymeric ¯uorocarbon compounds [6±9]. How-
ever, degradation products were not detected by infrared
spectral analysis or gas chromatography for BisOSBP-A-
TFE. Furthermore, total acid number (TAN) of post-test
sample of BisOSBP-A-TFE showed no change, clearly
indicating that the scission observed with per¯uoropo-
lyethers did not occur. The authors believe that electrons
from the benzene rings strengthened the adjacent carbon±
oxygen bonds, suppressing the scission reaction catalyzed
by iron ¯uoride.
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[4] S. Mori, J. Jpn. Soc. Tribologists 33 (1988) 585.
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4. Conclusions
These results suggest the following conclusions: (1) the
basic molecular structure of the ¯uorinated alkyl aryl ether